Porcine circovirus type 2 (PCV2) is a small (17-22 nm in diameter), icosahedral, non-enveloped DNA virus, which contains a single-stranded circular genome. PCV2 shares approximately 80% sequence identity with porcine circovirus type 1 (PCV-1). However, in contrast with PCV1, which is generally non-virulent, swine infected with PCV2 exhibit a syndrome commonly referred to as Post-weaning Multisystemic Wasting Syndrome (PMWS). PMWS is clinically characterized by wasting, paleness of the skin, unthriftiness, respiratory distress, diarrhea, icterus, and jaundice. In some affected swine, a combination of all signs will be apparent while other swine will only have one or two of these clinical signs. During necropsy, microscopic and macroscopic lesions also appear on multiple tissues and organs, with lymphoid organs being the most common site for lesions. A strong correlation has been observed between the amount of PCV2 nucleic acid or antigen and the severity of microscopic lymphoid lesions. Mortality rates for swine infected with PCV2 can approach 80%. In addition to PMWS, PCV2 has been associated with several other infections including pseudorabies, porcine reproductive and respiratory syndrome (PRRS), Glasser's disease, streptococcal meningitis, salmonellosis, postweaning colibacillosis, dietetic hepatosis, and suppurative bronchopneumonia.
Currently, there are three subtypes of PCV2 known (PCV2a, PCV2b and PCV2c), which are classified according to a unified nomenclature for PCV2 genotypes (Segales, J. et al., 2008, PCV-2 genotype definition and nomenclature, Vet Rec 162:867-8). Two further subtypes (PCV2d and PCV2e) have been proposed (Wang et al. Virus Res. 2009 145(1):151-6) but, however, it was demonstrated later that they belong to the PCV2a and PCV2b clusters (Cortey et al. Vet Microbiol. 2011 149(3-4):522-32011). According to this unified nomenclature for PCV2 genotypes the orf2 gene is used to perform genotyping for pcv-2, wherein the geotyping is based on the proportion of nucleotide sites at which two sequences being compared are different (p distance). This value is obtained by dividing the number of nucleotide differences by the total number of nucleotides compared (Kumar et al. 2001 Bioinformatics 17, 1244-1245) and subsequently, the construction of a p distance/frequency histogram enables to determine potential cut-off values to distinguish different genotypes (Rogers and Harpending 1992 Molecular Biology and Evolution 9, 552-569; Biagini et al. 1999 Journal of General Virology 80, 419-424). Using this methodology, orf2 pcv-2 sequences are assigned to different genotypes when the genetic distance between them is 0·035.
US 2011/0305725 A1 describes a study planned to test a new vaccine formulation in pigs to assess its efficacy against porcine circovirus and M. hyopneumoniae. During the course of this study, it was observed that several of the pigs in the control and vaccinated groups exhibited clinical signs of PMWS. It was then confirmed that these pigs were exposed to environmental PCV2 prior to challenge. Molecular analysis on blood and tissue samples from these pigs revealed that they harbored a type 2B strain that was different than the strain used for challenge (paragraph [0152] of US 2011/0305725 A1).
WO2011116094 A2 discloses a chimeric porcine circovirus infectious DNA clone and live attenuated chimeric virus with the PCV2 of subtype PCV2b, and a capsid gene of subtype PCV2b integrated into a non-pathogenic PCV1 virus genome, wherein the attenuated chimeric virus can be used as a live vaccine, as well as an inactivated (killed) vaccine.
WO2013030320 A1 relates to synthetic Circovirus type capsid proteins and to methods for treating and/or preventing PCV2-associated diseases in mammals using said proteins. Two sequences were designed according to WO2013030320 A1, wherein one sequence was modified further with, among others, the following optimizations:                A potential cleavage site was eliminated at amino acid position 165.        A mutation was introduced in position 200.        A replacement was made in position 161.        A replacement was made in position 170.        The S residue in position 225 was replaced with a D.        A replacement was made at position 143.        Two replacements were made at the N-terminal of the sequence (positions 13 and 20).        
However, as in practice the expression of wild type PCV2b ORF2 protein is found to be insufficient and requires further concentration steps in order to receive virus like particles (VLPs) useful to prepare a subunit vaccine, an easy modification of naturally occurring PCV2b ORF2 protein sequences is needed for enhancing the expression efficacy and for increasing the production of VLPs, thereby allowing a fast and easy production of effective PCV2 subunit vaccines.
The solution to the above technical problem is achieved by the description and the embodiments characterized in the claims.
Thus, the invention in its different aspects is implemented according to the claims.